1. Field of the Invention
The invention relates to radiological image intensifier tubes (RII tubes) using a vacuum electron tube and a luminescent observation screen.
The luminescent observation screen of an RII tube has the role of generating a visible light image when the screen is excited by an electron beam that strikes its photoluminescent surface. The invention relates more particularly to the luminescent observation screen comprising means to improve the contrast of the image.
Image intensifier tubes are vacuum tubes comprising an input screen located at the front of the tube, an electronic optical system and an image observation screen located at the rear of the tube, on the side of an output window of this tube.
In radiological image intensifier tubes, the input screen furthermore has a scintillator screen that converts the incident X photons into visible photons.
The visible photons excite a photocathode which, in response, generates an electron flux. This electron flux is then transmitted by an electronic optical system that focuses the electrons and directs them to the observation screen. The observation screen has one or more layers of luminophore grains deposited on a transparent glass support. The luminophores struck by the electrons then produce visible light from the exterior of the tube through the transparent support.
2. Description of the Prior Art
FIG. 1 gives a schematic view of a radiological type of image intensifier tube of this kind.
The intensifier tube 1 has a glass casing 2 of which one end, in front of the tube, is closed by an input window 3 exposed to radiation by X photons.
The second end of the casing forming the rear tube is closed by the observation screen 4 which is transparent to light.
The X-rays are converted into visible light rays by a scintillator screen 5. The visible light rays excite a photocathode 6 which, in response, produces electrons. These electrons are extracted from the photocathode 6 and accelerated to the observation screen 4 by means of different electrodes 7 and an anode 8 positioned along a longitudinal axis 9 of the tube and forming the electronic optical system.
In the example shown, the observation screen 4 is formed by a transparent element made of glass attached in an imperviously sealed manner to the casing 2. This glass element furthermore, in the example shown, comprises a support that bears luminophores 10 for example.
In certain embodiments of image intensifier tubes, the setting up of the acceleration potential of the electrons coming from the photocathode of the tube is done by a grid, under the acceleration voltage, that is positioned in the vicinity of the observation screen. The accelerated electrons go through the grid reaching the luminophores 10, of the observation screen, which produce visible light.
In other embodiments, the acceleration potential is obtained by a voltage applied to a thin layer of conductive material for example a metal deposited directly on the layer of luminophores of the observation screen. The small thickness of this metal layer enables the electrons to pass to the luminophores without great loss. In present embodiments, the thickness of the metal layer, which is made of aluminium for example, is about 0.3 micrometers.
This very small thickness is chosen so that the layer is transparent to electrons, in order that these electrons may reach the photocathode without losses. Indeed, aluminum is not naturally transparent to electrons except in very small thicknesses.
Furthermore, it has already been proposed, for cathode-ray tubes and not for radiological image intensifier tubes, to coat the aluminum layer with a carbon layer in order to limit the production of secondary electrons by the luminophore layer. For, carbon is a material with a very low secondary emission coefficient. Therefore, it does not generate excessive secondary electrons when it is struck by the electrons of the incident beam and furthermore it is relatively transparent to these incident electrons (which are high-energy electrons) but is absorbent to the secondary electrons which are transmitted by the layer of luminophores.
The emission of secondary electrons is inconvenient for the following reason. In the image intensifier tubes, the accelerated electrons coming from the photocathodes strike the layer of luminophores prompting an emission of photons in the visible light domain representing the radiological image. If we consider a beam of incident electrons Ei (see FIG. 1) striking the luminophores in a zone Z of the observation screen, these luminophores in turn, by the impact of the incident electrons Ei, produce re-emitted electrons Er that are directed towards the interior of the tube and then again fall on the luminophores of the observation screen at a place which is not the place at which they were emitted. This phenomenon is parasitic with respect to the initial image produced by the beam of incident electrons Ei. Hereinafter, these re-emitted electrons shall be called back-scattered electrons.
In present-days structures, the back-scattered electrons represent about 20% of the beam of incident electrons EI and are highly parasitic with respect to the image produced by the observation screen. The back-scattered electrons get dispersed when they are emitted into the tube. When they fall on the observation screen, accelerated by the bias voltages of the tube, these back-scattered electrons excite the luminophores of the observation screen in a totally distributed way. This secondary phenomenon produces a background noise expressed by a reduction of the contrast of the image.
In an unexpected way, it has been perceived that it is possible to obtain an equally efficient result without depositing any carbon layer, by using solely the properties of aluminium and choosing an aluminum layer thickness that is far greater than what normally ought to be chosen, given the low transparency of this material to electrons.
The fact is that aluminium has a secondary emission coefficient low enough for it to play partly the same role as the carbon layer. The aluminum layer may then constitute a single luminophore coating layer. The manufacture of the RII tube is thereby simplified.